LEDs are a desirable choice of light source in part because of their relatively small size, low power/current requirements, rapid response time, long life, robust packaging, variety of available output wavelengths, and compatibility with modern circuit construction. These characteristics may help explain their widespread use over the past few decades in a multitude of different end use applications. Improvements to LEDs continue to be made in the areas of efficiency, brightness, and output wavelength, further enlarging the scope of potential end-use applications.
LEDs are typically sold in a packaged form that includes an LED die or chip mounted on a metal header. The header can have a reflective cup in which the LED die is mounted, and electrical leads connected to the LED die. Some packages also include a molded transparent resin that encapsulates the LED die. The encapsulating resin can have either a nominally hemispherical front surface to partially collimate light emitted from the die, or a nominally flat surface. Other materials besides resins have been proposed for the encapsulating body, referred to herein as an encapsulant. For example, U.S. Pat. No. 3,596,136 (Fischer) discusses LEDs having domes made of certain glasses, glasses comprising by weight 19 to 41% arsenic, 10 to 25% bromine, and either 28 to 50% sulfur or 65 to 70% selenium. Fischer reports at least one glass that is yellow in color with a refractive index of about 2.4, another glass that is red in color with a refractive index between 2.5 and 2.7, and still another glass that is black in color with a refractive index of about 2.9.
It is also known to utilize an optical element that is made separately and then brought into contact or close proximity with a surface of an LED die to couple or “extract” light therefrom and reduce the amount of light trapped within the die. Such an element is referred to herein as an extractor. Extractors normally have an input surface sized and shaped to substantially mate with a major emitting surface of the LED die.
LEDs generate light within high refractive index semiconductor materials that make up the die of the LED. If the die is immersed in air, the large refractive index mismatch between the semiconductor and air causes much of the light propagating within the die to be totally internally reflected at the die/air interface. Only light traveling at angles within a relatively narrow escape cone associated with the interface can refract into the air and escape the die. The half-angle of the escape cone is the well-known critical angle for the interface. As a result, much of the light generated by the die is wasted, and the achievable brightness of the LED suffers.
Both encapsulants and extractors can be used to reduce the amount of wasted light and improve brightness. They do this by providing a light-transmissive material at the surface of the LED die whose refractive index (n) is closer to that of the die than air, reducing the refractive index mismatch at the interface and increasing the span of the escape cone. The closer n is to the refractive index of the die, the less light is wasted inside the die, and the brighter the LED can shine.
From a practical standpoint, conventional encapsulants have been successful to only a limited extent in this regard. The encapsulant substantially surrounds the die, and because of this and the large temperature shifts from the heat generated at the die, the encapsulant material is selected not only for its refractive index properties but also for its thermal and mechanical properties to avoid damaging the LED die over many temperature cycles, and for its ability to resist yellowing or other degradation when exposed to the high flux emitted by the die. As a result, most encapsulated LEDs utilize specialized epoxy resins that have a refractive index n of only about 1.4 to 1.6. These values are well above the refractive index of air (n=1), but well below that of most LED dies (n≈2.3 or higher). Thus, there is still room for substantial improvement.
Extractors are currently not as widely used in LEDs as encapsulants, possibly due to additional manufacturing steps needed to first fabricate the extractor and then hold it in position at the LED die, and the associated expense and complexity. These problems are exacerbated by the small sizes involved, with LED dies typically having a characteristic dimension on the order of millimeters or less.